1. Field of the Invention
The present invention relates to compositions and methods for drilling, cementing and casing boreholes in subterranean formations, particularly hydrocarbon bearing formations. More particularly, the present invention relates to methods for improving the rheology of invert emulsion drilling fluids, particularly at broad temperature ranges such as encountered in deep water, and to compositions for invert emulsion drilling fluids with good stability and high performance properties.
2. Description of Relevant Art
A drilling fluid or mud is a specially designed fluid that is circulated through a wellbore as the wellbore is being drilled to facilitate the drilling operation. The various functions of a drilling fluid include removing drill cuttings from the wellbore, cooling and lubricating the drill bit, aiding in support of the drill pipe and drill bit, and providing a hydrostatic head to maintain the integrity of the wellbore walls and prevent well blowouts.
An important property of the drilling fluid is its rheology, and specific rheological parameters are intended for drilling and circulating the fluid through the well bore. The fluid should be sufficiently viscous to suspend barite and drilled cuttings and to carry the cuttings to the well surface. However, the fluid should not be so viscous as to interfere with the drilling operation.
Specific drilling fluid systems are selected to optimize a drilling operation in accordance with the characteristics of a particular geological formation. Oil based muds are normally used to drill swelling or sloughing shales, salt, gypsum, anhydrite and other evaporate formations, hydrogen sulfide-containing formations, and hot (greater than about 300 degrees Fahrenheit (“° F.”) holes, but may be used in other holes penetrating a subterranean formation as well.
An oil-based or invert emulsion-based drilling fluid may commonly comprise between about 50:50 to about 95:5 by volume oil phase to water phase. Such oil-based muds used in drilling typically comprise: a base oil comprising the external phase of an invert emulsion; a saline, aqueous solution (typically a solution comprising about 30% calcium chloride) comprising the internal phase of the invert emulsion; emulsifiers at the interface of the internal and external phases; and other agents or additives for suspension, weight or density, oil-wetting, fluid loss or filtration control, and rheology control. In the past, such additives commonly included organophilic clays and organophilic lignites. However, recent technology as described for example in U.S. Pat. Nos. 7,462,580 and 7,488,704 to Kirsner, et al., introduced “clay-free” invert emulsion-based drilling fluids, which offer significant advantages over drilling fluids containing organophilic clays.
As used herein and for the purposes of the present invention, the term “clay-free” (or “clayless) means a drilling fluid formulated without addition of any organophilic clays or organophilic lignites to the drilling fluid composition. During drilling, such “clay-free” drilling fluids may acquire clays and/or lignites from the formation or from mixing with recycled fluids containing clays and/or lignites. However, such contamination of “clay-free” drilling fluids is preferably avoided and organophilic clays and organophilic lignites should not be deliberately added to “clay-free” drilling fluids during drilling.
Invert emulsion-based muds or drilling fluids (also called invert drilling muds or invert muds or fluids) comprise a key segment of the drilling fluids industry, and “clay-free” invert emulsion-based muds, particularly those capable of “fragile gel” behavior as described in U.S. Pat. Nos. 7,462,580 and 7,488,704 to Kirsner, et al., are becoming increasingly popular. “Fragile gel” behavior or “fragile gel” strength generally refers to the ability of the drilling fluid to both suspend drill cuttings at rest and show a lack of a pressure spike upon resumption of drilling.
Clay-free invert emulsion drilling fluids, like INNOVERT® drilling fluid available from Halliburton Energy Services, Inc., in Duncan, Okla. and Houston, Tex., for example, have been shown to yield high performance in drilling, with “fragile gel” strengths and rheology leading to lower equivalent circulating density (ECDs) and improved rate of penetration (ROP).
A limiting factor in drilling a particular portion of a well is the mud weight (density of the drilling fluid) that can be used. If too high a mud weight is used, fractures are created in lost-circulation zones with resulting loss of drilling fluid and other operating problems. If too low a mud weight is used, formation fluids can encroach into the well, borehole collapse may occur due to insufficient hydrostatic support, and in extreme cases safety can be compromised due to the possibility of a well blowout. Many times, wells are drilled through weak or lost-circulation-prone zones prior to reaching a potential producing zone, requiring use of a low mud weight and installation of sequential casing strings to protect weaker zones above the potential producing zone. A particularly critical drilling scenario is one that combines deepwater and shallow overburden, as is typical of ultra deepwater fields in Brazil. This scenario is characterized by high pore fluid pressure, low effective stresses, low fracturing gradients and narrow mud weight windows.
Commercially available clay-free invert emulsion drilling fluids may have less than preferred rheology at low mud weights, that is, mud weights ranging from about 9 ppg to about 12 ppg, with temperatures up to about 375° F. or higher. Addition of inert solids may improve the rheology, but result in a decreased rate of penetration during drilling and loss of or decline in other benefits seen with a clay free system. Such inert solids include for example, fine sized calcium carbonate, and the term as used herein is not meant to be understood to include or refer to drill cuttings. Low mud weight or reduced density clay-free oil based invert emulsion drilling fluids also may show a decline in the desired “fragile gel” strength characteristic of clay-free invert emulsion drilling fluids.
Conventionally, low rheology invert emulsion drilling fluids are used for deepwater drilling at cold temperatures (typically about 40° F.). Thinners are typically added to an invert emulsion drilling fluid for use in deepwater cold temperatures (typically about 40° F.), to prevent a substantial increase in the rheology of the fluid. Low rheology, however, affects the drilling fluid's ability to clean the borehole and the fluid's ability to suspend barite and drill cuttings. An ideal deepwater invert emulsion drilling fluid exhibits sufficiently high and similar rheology from the rig floor to the riser and along the depth of the well. Such an invert emulsion drilling fluid is typically called a “flat rheology drilling fluid” or a “flat rheology fluid.”
An invert emulsion drilling fluid having and maintaining a similar rheology along the depth of a borehole would lead to controlled equivalent circulating densities (ECDs) along the depth of the well-bore. The improved flow properties of such a fluid having similar or flat rheology would reduce the equivalent circulating density (ECD) and subsequent losses of the fluid to induced fractures in the subterranean formation. The yield point (YP) of the fluid would be easily maintained and this would contribute to efficient cuttings clearance. There would be a reduced pressure drop in the drill pipe and increased pump output in using such a fluid, which would also improve hole cleaning and reduce the cuttings load. Further, such a fluid, with a similar YP and low-shear yield point (LSYP), would also ensure the lowest possible frictional losses in the annulus and minimum hydraulic contribution to ECD. Reducing the hydraulic contribution to ECD reduces the risk of exceeding the fracture gradient of the subterranean formation
Increasingly invert emulsion-based drilling fluids are being subjected to ever greater performance and cost demands as well as environmental restrictions. Consequently, there is a continuing need and industry-wide interest in new drilling fluids that provide improved performance while still affording environmental and economical acceptance, particularly for deepwater drilling applications.